First off, I don't know what kind of a.m. skiing you like (short vs. long radius turns) and what your ability is, but I would put your size range at 174- 183 cms for an all around ski. If you plan to do more "dececnt sized kickers" or powder , go longer, more rails and maybe moguls, then go shorter.
For what you mentionned, I would recomend the Line kryptonites over the regular chronics becuase the wider waist will be more versatile out of the park. For 4frnt, the MSP may be a more all-around ski than the STL, again depending on how much park or a.m. Aramada AR6 may be another good option. Best of luck, KiP